Soils, mud, dirt Sticking to glass?

First of all, please feel free to move this question wherever it should go. I truly have no idea if I'm looking in the right place at all, just a point in the right direction would be greatly appreciated.

My question:

For a project I'm working on, I'm required to flow a sample of soils/mud/clay/dirt suspended in either water or a buffer through a glass tube horizontally. Of course, often gravity has it's way and particles sink to the bottom and stick to the bottom of the tube. This is of course expected with big particles, but smaller particles (to give an idea of scale - less than microns) should be pushed along by the flow of the solution? However, they still stick to the bottom. My question is - what are the forces involved? Electrostatic charge? Or is it just good old gravity? It's way out of my area, so as I say, any help, any point in the direction would be greatly appreciated.

It's electrostatic. There's a couple quick fixes to try: 1) instead of water, try a salt buffer: the salt will screen the charges, 300 mM NaCl or KCl is a good starting point. 2) I use fetal bovine serum to help prevent PS or PMMA microspheres from sticking to glass surfaces; the proteins in FBS seem to stick first (via electrostatic interactions), and that helps the particles from sticking.

It's not a trivial problem to solve, if that helps. Sonication may be useful, but may not be practical for your application.